When electrical devices are connected to current supplies these supplies may experience large voltage fluctuations or have superposed on the current supply large transient voltages which interfere with the operation of control devices which are connected to the current supply. An example of the foregoing is the situation of a mobile transmitter or receiver employed for radio telephone service while installed in a motor vehicle or ship. These mobile installations are connected to a direct current source such as a battery from which operating current is drawn.
A typical mobile telephone installation in a car may comprise a radio telephone set, an operating device and a talking location. The radio telephone device consists of a transmitter, a receiver, and a combined selective calling and dial system (SRWE). The SRWE system functions in a manner designed to control the making and breaking of the telephone connection as well as to perform control functions which are required for operation of the radio telephone installation. The radio telephone is generally installed in the trunk of a motor vehicle and is connected by a cable with the talking and operating location. The radio telephone installed in the trunk is also connected to the battery of the vehicle by another cable.
The selective calling and dial system function may be performed by a microprocessor which is utilized in the following manner. The code conversion of the coded numbers transmitted over the radio channel are stored in fixed valve memories (ROM). ROM is the acronym for read only memories, which memories store preselected information to be recalled when required. Information which is processed and which changes such as a calling number or the channel group of a party is stored in a random access memory (RAM).
The ROM memory will store its information indefinitely whether power is applied or removed from the memory. The RAM memory, on the other hand, requires that it be refreshed by the microprocessor in order to retain its information. Removal of power from the microprocessor or the RAM will result in a loss of information stored in the memory. Consequently the microprocessor and RAM will be provided with an alternate source of current. This alternate source of current is provided by an internal battery when current from the vehicle battery is turned off.
Further, during switching of the various control functions transient conditions may occur which are superposed on the direct current supply and provide over voltage or under voltage conditions to the microprocessor and RAMs. During these transient conditions the microprocessor may be caused to execute incorrect orders and to write or cancel data stored in the RAMs. Therefore, the radio telephone equipment controlled by the microprocessor will malfunction with the placement of incorrect calls as a possibility.
Prior art has attempted to correct for the transient conditions as well as over or under voltage conditions which may exist at different instances of time, these fluctuations of voltage will interfere with the proper functioning of the radio telephone system as herein before described.
One of the prior art devices which is used to eliminate the over voltage condition is the zener diode. The zener diode is disposed across the direct current supply in series with a current limiting resistor. The regulated current is obtained from the zener diode. The zener diode will clip transient voltages which are in excess of the zener diode value. Therefore voltage transients due to switching processes such as obtained when starting the engine of the motor vehicle will be stabilized. The zener diode will not be able to regulate the supply voltage during the times when the voltage falls below the zener diode level.
When the radio telephone is turned off the removal of voltage from the microprocessor and RAMs may be delayed by the use of large electrolytic capacitors. These large capacitors store the electrical energy and change the stored energy at a slow rate. The capacitors are a prior art device which is used to compensate for under voltage conditions for short periods of time.
Prior art has also employed a switching over to an auxiliary voltage source to power the microprocessor and the RAMs when the radio telephone has been shut off by removal of vehicle battery current. For this purpose a switch may be provided which is manually operated so that when the radio telephone is turned off auxiliary current may be supplied. There is a great probability that the switch may not be operated. Furthermore, the requirement exists that the switch be operated at a proper time after the under or over voltage condition exists.
Much of the aforementioned prior art has appeared in various publications which follow and are incorporated herein by reference. From Elektronik, 1967, Heft 7, Dipl.-ing. Manfred Hanemann "Elektronische Langzeitglieder", electronic time circuits for obtaining long delay times or switch times are known. Thereby, the time defining capacity is charged linear. For realizing a switch on delay circuit, a circuit arrangement is mentioned on page 208, 2.4.2. "Einschaltverzoegerung", wherein a transistor only switches when at the capacity the zener voltage of a zener diode is obtained in the emitter circuit of this transistor. On page 209, 2.4.3. "Abschaltverzoegerung", a circuit arrangement is mentioned wherein a condenser is dicharged linear through a transistor and a resistor. Only after the delay time which is adjusted by means of the condenser, the resistor and a diode, the input side potential charge is indicated at the output.
These electronic time circuits have the disadvantage that a relative expensive circuit arrangement has to be used for obtaining short delay times. If, as stated on page 207, a monostable flip-flop is used as a time member, already interference dimensions like current fluctuations or interference pulses at the output of this time member may be triggered. Furthermore, on page 209 a discriminator circuit is mentioned with a zener diode in the base circuit of a transistor. In this circuit arrangement it is disadvantageous that the threshold voltage is exclusively determined by the zener voltage, and that in view of the exemplary fluctuations of the same zener diodes different threshold voltages are generated.